In accordance with a motion compensation-transform coding image compression technique, such as H.264 or high efficiency image coding (HEVC), transform is always performed. However, the degree that video energy is concentrated on a low frequency is not great although transform is performed if the degree of change of a spatial pixel value within a corresponding block whose video will be compressed is very large or sharp. In a compression process, if a low frequency component is maintained and a high frequency component is removed or strong quantization is applied to the high frequency component, picture quality may be severely deteriorated. In particular, in the case of an artificial video produced by a computer, such a problem is significant. In this case, if transform is not performed and coding is performed in a spatial domain rather than performing transform uniformly as in an existing method, compression performance becomes better or the deterioration of picture quality can be significantly reduced. In the existing video compression methods, however, transform needs to be always performed regardless of the properties of video. If the existing video compression methods are applied, there is always a problem in that picture quality is deteriorated or a compression ratio is reduced.
In order to avoid such a problem, it may be considered to insert a flag indicative of whether or not to skip transform into compressed data so that whether or not to skip transform for each transform block is indicated. If the flag is sent for each transform block, however, another problem is generated because overhead consumed for signaling is increased.
As a solution for such a problem, there has been known a method for signaling a flag to determine whether or not to skip transform only when a current transform block size (TU size) is 4×4. In such a technology, transform is not skipped in a transform block greater than 4×4, and the execution of transform is selectively skipped only if a transform block is 4×4. Accordingly, a flag is sent only in the case of the 4×4 transform block. If the recently developed RQT technology is used, a transform block size suitable for each block can be determined adaptively and hierarchically using an RQT transform technology of multiple sizes divided in a quad tree. In such a technology, if it is determined that the effectiveness of transform is small as a result of the review of the characteristics of video within a specific block, such an inefficiency problem can be limited to some transform blocks by splitting a block into finer transform block sizes and performing transform. Since the smallest transform size is 4×4, split transform is no longer performed if a transform block size that is now taken into consideration is 4×4. Accordingly, even in this case, whether or not to skip transform is determined and a flag indicative of a result of the determination is transmitted so that a decoder is aware of whether or not to skip the execution of transform. In this case, there are advantages in that overhead consumed for a flag can be reduced and transform is skipped if the effectiveness of the transform is low.
Meanwhile, in current video compression technologies, each encoder autonomously may adaptively determine the smallest transform block size to be used by the decoder. In a specific application, the smallest transform block size may be determined to be another size, such as 8×8 or 16×16, other than 4×4. If the smallest transform block size (hereinafter referred to as an STU) determined by an encoder is 8×8 or more, a current technology has a technical disadvantage in that the execution of block transform cannot be skipped even under any circumstances. That is, in this case, since transform efficiency of a block within video is very low, a reduction of a compression ratio or the deterioration of picture quality must be endured because there is a problem in that transform cannot be skipped if a current technology is used although it is more advantageous to skip transform.
Meanwhile, the size of a transform block having the inefficiency problem of transform may be different in various ways depending on the characteristics of video. The inefficiency problem of transform may be commonly generated in the 4×4 transform block because the smallest transform block size is 4×4, but the inefficiency problem may be generated in an 8×8, 16×16, or 32×32 transform block size depending on the characteristics of video. That is, a problem, such as a reduction of a compression ratio or the deterioration of picture quality in some video, may be generated if the inefficiency problem is taken into account only in the 4×4 transform block and transform is skipped according to an existing technology.